4 “Blessed are the meek, for they shall by inheritance possess the earth:” that earth, I suppose, of which it is said in the Psalm, “You are my refuge, my portion in the land of the living.” For it signifies a certain firmness and stability of the perpetual inheritance, where the soul, by means of a good disposition, rests, as it were, in its own place, just as the body rests on the earth, and is nourished from it with its own food, as the body from the earth. This is the very rest and life of the saints. Then, the meek are those who yield to acts of wickedness, and do not resist evil, but overcome evil with good. Let those, then, who are not meek quarrel and fight for earthly and temporal things; but “blessed are the meek, for they shall by inheritance possess the earth,” from which they cannot be driven out.
5. “Blessed are they that mourn: for they shall be comforted.” Mourning is sorrow arising from the loss of things held dear; but those who are converted to God lose those things which they were accustomed to embrace as dear in this world: for they do not rejoice in those things in which they formerly rejoiced; and until the love of eternal things be in them, they are wounded by some measure of grief. Therefore they will be comforted by the Holy Spirit, who on this account chiefly is called the Paraclete, i.e. the Comforter, in order that, while losing the temporal joy, they may enjoy to the full that which is eternal.
6. “Blessed are they which do hunger and thirst after righteousness: for they shall be filled.” Now He calls those parties, lovers of a true and indestructible good. They will therefore be filled with that food of which the Lord Himself says, “My meat is to do the will of my Father,” which is righteousness; and with that water, of which whosoever “drinks,” as he also says, it “shall be in him a well of water, springing up into everlasting life.”
7. “Blessed are the merciful: for they shall obtain mercy.” He says that they are blessed who relieve the miserable, for it is paid back to them in such a way that they are freed from misery.
8. “Blessed are the pure in heart: for they shall see God.” How foolish, therefore, are those who seek God with these outward eyes, since He is seen with the heart! As it is written elsewhere, “And in singleness of heart seek Him.” For that is a pure heart which is a single heart: and just as this light cannot be seen, except with pure eyes; so neither is God seen, unless that is pure by which He can be seen.
9. “Blessed are the peacemakers: for they shall be called the children of God.” It is the perfection of peace, where nothing offers opposition; and the children of God are peacemakers, because nothing resists God, and surely children ought to have the likeness of their father. Now, they are peacemakers in themselves who, by bringing in order all the motions of their soul, and subjecting them to reason— i.e. to the mind and spirit— and by having their carnal lusts thoroughly subdued, become a kingdom of God: in which all things are so arranged, that that which is chief and pre-eminent in man rules without resistance over the other elements, which are common to us with the beasts; and that very element which is pre-eminent in man, i.e. mind and reason, is brought under subjection to something better still, which is the truth itself, the only-begotten Son of God. For a man is not able to rule over things which are inferior, unless he subjects himself to what is superior. And this is the peace which is given on earth to men of goodwill; this the life of the fully developed and perfect wise man. From a kingdom of this sort brought to a condition of thorough peace and order, the prince of this world is cast out, who rules where there is perversity and disorder. When this peace has been inwardly established and confirmed, whatever persecutions he who has been cast out shall stir up from without, he only increases the glory which is according to God; being unable to shake anything in that edifice, but by the failure of his machinations making it to be known with how great strength it has been built from within outwardly. Hence there follows: “Blessed are they which are persecuted for righteousness' sake: for theirs is the kingdom of heaven.”
Source: Our Lord's Sermon on the Mount (New Advent)